1. Field of the Invention
This invention pertains to process parameter sensors and methods of operation therefor, and more particularly, to vibrating conduit parameter sensors and methods of operation therefor.
2. Statement of the Problem
Coriolis effect mass flowmeters are commonly used to measure mass flow and other information for materials flowing through a conduit. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. No. 4,109,524 of Aug. 29, 1978, U.S. Pat. No. 4,491,025 of Jan. 1, 1985, and RE 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters typically include one or more conduits having a straight or a curved configuration. Each conduit may be viewed as having a set of vibration modes, including, for example, simple bending, torsional, radial and coupled modes. In a typical mass flow measurement application, each conduit is driven to oscillate at resonance in one of its natural modes as a material flows through the conduit. The vibration modes of the vibrating, material-filled system are effected by the combined mass and stiffness characteristics of the conduits and the characteristics of the material flowing within the conduits.
A typical component of a Coriolis flowmeter is the drive or excitation system. The drive system operates to apply a periodic physical force to the conduit that causes the conduit to oscillate. The drive system typically includes at least one actuator mounted to the conduit(s) of the flowmeter. The actuator typically comprises one of many well known electromechanical devices, such as a voice coil device having a magnet mounted to a first conduit and a wire coil mounted to a second conduit, in an opposing relationship to the magnet. A drive circuit continuously applies a periodic, e.g., a sinusoidal or square wave, drive signal to the actuator coil. The periodic drive signal causes the actuator to drive the two conduits in an opposing periodic pattern that is thereafter maintained.
When there is effectively xe2x80x9czeroxe2x80x9d flow through a driven flowmeter conduit, points along the conduit tend to oscillate with approximately the same phase or a xe2x80x9czero-flowxe2x80x9d phase with respect to the driver, depending on the mode of the driven vibration. As material begins to flow from an inlet of the flowmeter, through the conduit and out of an outlet of the flowmeter, Coriolis forces arising from the material flow tend to induce phase shifts between spatially separate points along the conduit. Generally, as material flows through the conduit, the phase on the inlet side of the conduit lags the driver, while the phase on the outlet side of the conduit leads the driver. The phase shift induced between two locations on the conduit is approximately proportional to the mass flow rate of material through the conduit.
Motion of a Coriolis flowmeter can be modeled as a superposition of a plurality of vibrational modes, i.e., as motion of a plurality of independent single degree of freedom (SDOF) systems, a respective SDOF system being characterized by a natural frequency and a damping. A typical one of the modes present in a dual-conduit Coriolis flowmeter is the first out-of-phase bending mode, e.g., a bending mode in which the two tubes of the Coriolis flowmeter vibrate in opposition to one another. Other modes may also be identified in a typical vibrating conduit Coriolis flowmeter, including a first out of phase twist mode which is generally attributable to Coriolis forces generated by material passing through the conduits, as well as in-phase, lateral and other vibrational modes excited by the actuator of the flowmeter and other vibrational sources coupled to the flowmeter.
Techniques have been proposed to produce drive signals that produce a desired modal response. U.S. patent application Ser. No. 08/890,785 to Cunningham, assigned to the assignee of the present application, describes derivation of force projection coefficients that produce a drive signal for an actuator that preferentially excites a desired vibrational mode. A related United States Patent Application entitled xe2x80x9cGeneralized Modal Space Drive Control System for a Vibrating Tube Process Parameter Sensor,xe2x80x9d to Cunningham, filed on Feb. 25, 1998 and also assigned to the assignee of the present application, describes generating drive signals for a number of actuators to preferentially excite one or more desired modes of a Coriolis flowmeter conduit.
The above-referenced techniques typically use a predetermined modal filter to generate force projection coefficients. The predetermined modal filter is typically generated offline using classic experimental modal analysis techniques or finite element modeling. The force projection coefficients may then be determined from the modal filter using trial and error, inverse modal transformation or other techniques.
Unfortunately, the predetermined modal filter may not take into account changes in sensor characteristics that may occur over time. In addition, conventional techniques may not account for in situ effects arising from interactions between the sensor and the material processing system in which it is operated. For example, additional modes may be excited by structural coupling with the material processing system or by vibration sources in the material processing system, such as pumps.
In light of the foregoing, it is an object of the present invention to provide self-characterizing vibrating conduit type parameter sensors and methods of operation therefor which can provide a more accurate estimate of a process parameter, e.g., mass flow, of a material processing system.
It is another object of the present invention to provide vibrating conduit type parameter sensors and methods of operation therefor which can provide more accurate characterization of sensor behavior.
It is yet another object of the present invention to provide vibrating conduit type parameter sensors and methods of operation therefor which are capable of in situ characterization of sensor behavior.
These and other objects, features and advantages are provided according to the present invention by self-characterizing vibrating conduit type parameter sensors which include a modal parameter estimator configured to receive motion signals representing motion of the sensor conduit at a plurality of locations and operative to estimate a modal parameter, e.g., a modal filter parameter or a force projection parameter, from the received motion signals. According to a modified reciprocal modal vector (MRMV) aspect of the present invention, a modal filter parameter is estimated by exciting the conduit over a range of frequencies, generating a frequency response function (FRF) matrix over the range of frequencies, and generating an estimate of the modal filter parameter from the FRF matrix. The estimate of the modal filter parameter may be generated during an initialization or calibration procedure. According to an adaptive modal filtering aspect, an error of an estimated modal response of a modal filter with respect to a reference modal response is determined, and the error is used to generate a new estimate of a modal filter parameter for the modal filter. Modal filter parameters and force projection parameters can thus be adaptively updated while the sensor is in operation.
In particular, according to the present invention, a self-characterizing sensor for measuring a process parameter for a material processing system includes a conduit configured to contain material from the material processing system and a plurality of motion transducers operative to produce a plurality of motion signals representing motion at a plurality of locations on the conduit. A modal parameter estimator is configured to receive the plurality of motion signals and operative to estimate a modal parameter from the received plurality of motion signals. The modal parameter, e.g., a modal filter parameter or a force projection parameter, relates behavior of the conduit to behavior of a single degree of freedom (SDOF) system. A process parameter estimator is configured to receive the plurality of motion signals, responsive to the modal parameter estimator and operative to estimate a process parameter for the material processing system from the received plurality of motion signals according to the estimated modal parameter.
According to a modified reciprocal modal vector (MRMV) estimation aspect of the present invention, a sensor conduit is excited over a range of frequencies. A plurality of motion signals representing motion in response to the excitation is received, and a plurality of frequency responses for the plurality of locations over the range of frequencies is determined from the received plurality of motion signals. An estimate of a modal filter parameter for the sensor is generated from the determined plurality of frequency responses. A frequency response function (FRF) matrix is generated from the plurality of frequency responses. A pole corresponding to a mode of the sensor is identified and an SDOF vector generated from the identified pole. An estimate of a modal filter parameter is then generated from the estimated FRF matrix and the estimated SDOF vector.
According to an adaptive modal filtering aspect of the present invention, an excitation applied to a sensor conduit is determined and applied to a reference modal system to generate a reference modal response. A modal filter is applied to the plurality of motion signals representing motion in response to the excitation to generate an estimated modal response according to an estimate of the modal filter parameter. A new estimate of the modal filter parameter is generated based on an error of the estimated modal response with respect to the reference modal response. New estimates of the modal parameter may be iteratively generated until an error of the estimated modal response with respect to the reference modal response meets a predetermined criterion. Improved parameter sensors and operating methods therefor may thereby be provided.